Study variations played with the most occurring 3...a6.
I personally like the Classical or Cordel Variation very much as the black bishop moves to the strong square c5 and puts pressure on f2.

Most popular setup is 3...a6 4. Ba4 - see video

Ruy Lopez - Chess Games

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Press F-Key (or click e7 or d2 on top) = Flip Board

I recommend

If you don't know which variation to play as Black then I recommend the Open Defense. It has been played in world championships in the past and is solid.

You have always "something" as Black to counter the white ideas and the setup is fairly clear and straightforward. Replay game 66 above and others to see how this opening variation is played. See basic position below